The invention relates to a process and a device for the production of melamine by pyrolysis of urea.
In the high-pressure processes for the production of melamine, urea is reacted to give melamine by means of an endothermic liquid-phase reaction. The liquid melamine, depending on the pressure and temperature conditions in the reactor, additionally contains different amounts of dissolved NH3 and CO2, and condensation by-products and unreacted urea. The melamine thus obtained is then solidified, for example, by quenching with water or with ammonia, by sublimation with subsequent desublimation or by releasing the pressure under specific conditions.
The reactor used is customarily a tank reactor with a central pipe and heating elements arranged outside the central pipe, which provide the heat necessary for the reaction. These heating elements are pipe bundles, in which a salt melt circulates, arranged parallel to the central pipe. Urea and NH3 are introduced at the bottom of the reactor, impinge on a distributor plate which is located underneath the central pipe and react in the free space between the pipe bundles, in which melamine is already situated, with decomposition and evolution of gas to give melamine. In WO 99/00374, such a reactor is depicted schematically, the flow direction of the melt also being indicated such that the reaction mixture outside the central pipe flows upwards between the pipe bundles and separates there into off-gas and liquid melamine. The off-gas is removed at the top of the reactor, one part of the melamine melt is removed from the reactor via an overflow and the other part of the melamine melt flows downwards within the central pipe on account of gravity.
This previously used type of reactor, however, has the disadvantage that the pipe bundles, in particular in the case of relatively high urea throughputs, corrode relatively rapidly and therefore have to be frequently exchanged.
Unexpectedly, it has now been found that the corrosion rate of the salt melt pipes can be significantly lowered if the mixture of urea with melamine and its decomposition takes place not outside, but inside the central pipe. Contrary to the original assumption that the flow direction of the melamine melt is such as indicated in WO 99/00374, it has been found that the flow direction of the melamine melt in the arrangement according to the invention is exactly the reverse, the melt in fact flows upward within the central pipe and downward outside the central pipe.
The supply of heat necessary for the overall endothermic reaction takes place by means of the heating pipes arranged outside the central pipe during the movement of the melt downward, so that in the lower part of the reactor an approximately 3-30xc2x0 C., preferably 5-15xc2x0 C., higher temperature prevails than in the upper part. The fact that the melamine melt in the upper part of the reactor, where it is removed via an overflow, is colder than in the lower part means a further advantage compared with the arrangement according to WO 99/00374, since the melamine melt in the subsequent sections has to be cooled less, and the equilibrium position of the melt at the lower temperature is shifted in the direction of the melamine, so that fewer by-products are formed.
The invention accordingly relates to a process for the production of melamine by pyrolysis of urea in a high-pressure reactor having a vertical central pipe with formation of a melamine melt, which is characterized in that
the melamine melt circulating in the reactor mixes in the lower region of the reactor with a urea melt introduced into the reactor from below and optionally introduced NH3,
the reaction mixture formed, consisting essentially of melamine, NH3, CO2 and optionally reaction intermediates, flows upwards from below in the central pipe,
the reaction mixture formed emerges from the central pipe in the upper part of the central pipe,
the separation between melamine and off-gas takes place at the top of the reactor above the central pipe,
a part of the melamine emerging at the top from the central pipe flows downwards in the annular space between the central pipe and reactor wall and the remainder is expelled for further work-up,
the off-gases are expelled at the top of the reactor.